Magnesium retort



Dec. ll, 1945. E. E. ENslGN ETAL MAGNESIUM RETR'T Filed Aug. l2, 1942 n O 0 O De w o m S 2 d 2 l Le uwbk. vwmawr 0 e l Sw N -Patented Dec. 11, 1945 MAGNESIUM RETORT Elbert E. Ensign, Ypsilanti, and Frank G. sham,

Detroit, Mich., assignors-to Ford Motor Company, Dearborn, Mich., a corporation of Dela- Wale Application August 12, 1942,` Serial No. 454,580

(Cl. 26S-19) Claims.

'I'his invention relates to furnaces or retorts for the production or refining of metals at high temperatures and low pressures, with particular reference to furnaces for the reduction of magnesium ores at high temperatures and low pressures.

The object of this invention is to provide a furnace including a metal retort in which the reduction of magnesium under vacuum can be efliciently and economically performed. Another object of this invention is to provide a method by which retorts of ingot iron or other substantially pure iron normally readily oxidizable in furnace atmospheres may be used and their superior high-temperature strength characteristics utilized. Another object Vis to devise a furnace in which metal retorts may be maintained at uniform high temperatures without failure occurringfrom thermal strains, oxidation or loss of strength due to the temperatures prevailing.

The process of producing magnesium directly by the reduction of its ores under subatmospheric pressures has been greatly hampered by a lack of suitableequipment. The reduction of ores, such as magnesium oxide -or dolomite, either by carbon orsilicon, necessitates the use of temperatures well above 2,000 degrees F. However, present-day techniques have been unable to produce metals suitable for use in retorts able'to withstand such temperatures under subatmospherc'pressures for the length of time required inthe reduction process.

Nichrome alloys so far seem to have the best combined heat and oxidation-resisting properties but even these deteriorate and collapse on continuous heating at temperatures ranging above 2,100 degrees F. in a comparatively short time. In fact `the life of a retort at 2,400 degrees F.- is rarely more than several hours. Since the practical, direct reduction of magnesium is limited only by the restrictions imposed by available retort materials, the importance of this invention may readily be realized. The advantages, too, are directly reflected in the cost of operation of the process, in which, heretofore, retort cost has been a major item.

With these and other objects in w'ew, the invention consists of thev arrangement, construction and combination of the Various parts of the improved'device and the steps of the method disclosed, as described in the specification, claimed in the claims, -and illustrated in the accompanying drawing in which:v

Figure 1 is a diagrammatic view of a molten bath furnace,following this invention.

Figure 2 is a temperature-pressure chart com puted for the reduction of magnesium oxide by carbon-under vacuum, but illustrative, generally,

v of the thermal reduction process.

Figure 3 is a curve that shows the actual ternperature and yield relationship in the reduction of magnesium oxide by ferrosilicon under pressures in the range -150 microns.

At this point, reference is made to Figure 2, which Shows the computed temperature-pressure equilibrium equation obtained in the magnesium oxide carbon reaction in which although it will be subject to variation when other substances such as N2, H2 or C02 are present. Of course, specific values will diier when other materials are used in thev presence of other gases, but the general conditions remain the same and the curves may be taken as indicative, generally, of the thermal reduction process. Curve A shows the equilibrium condition and is expressed in partial pressure of magnesium vapor against temperature. It may be shown that the partial pressure of the carbon monoxide must equalthe vaporv pressure of Mg,v and accordingly curve B represents the permissive total pressure obtained in the system'for equilibrium conditions if no other gases are present.

Therefore, at all Apoints above the curve B, it will be seen-that at least some magnesium will be disassociated and present as a gas.v Moreover, it is seen that if magnesium vapors are to be had at very low temperatures, pressure must be extremely lowthat is, in order to obtain magnesium vapors at 2,000 degrees F., pressures not to exceed 20 microns must b e maintained, Thus far, in commercial production, it has been very diliicult to maintain these low pressures and practical operation, therefore, must be carried on at temperatures well above the minimum range. However, it is apparent that if retort equipment could stand higher temperatures, say around 2,400 degrees F., the present-day, 10W-pressure equipment would be more than adequate. In this invention, the reduction of magnesium ores at temperatures of about 2,400 degrees F. is accomplished and the life of the retort actually lengthened which makes this type of extraction economically and practically-feasible.

The curve in Figure 3 shows the percentage yield against temperature relationship in the reduction of a dolomitic ore .by ferrosilicon. This curve shows that vwith each increment in temperature the actual yield expressed in percentage of the yield theoretically obtainable correspondingly increases. Increase in temperature is not directly reflected in a proportional increase in yield, since the curve is not a straight line although the exact reason is not yet clear. yield is approximately constant and the curve follows a plateau between 2,000 an'd 2,100 degrees F., but the next increase of 100 degrees in temperature shows a sharp rise in yield. Again, be

tween 2,200 and 2,300 degrees the yield is constant but then rises sharply within the next 100 degrees and again levels off to another plateau. Curve C was obtained by running numerous tests on a briquetted, dolomitic, ferrosilicon mixture and varying tle temperatures while maintaining the pressure relatively constant at from 50-150 microns.

These theoretical considerations clearly show the necessity for operation at temperatures in excess of 2,100 degrees F. and pressures not toexceed 100 microns. Other data, not detailed here, show that yield is also a function of time of reaction which may vary from two to seventytwo hours. Eouating these, it will be observed that the conditions for maximum yield are mutually antagonistic to the conditions for retort life which requires temperatures not to exceed 2,000 degrees F. at the stated vacuum and minimum time of exposure to high temperature. Other considerations such as chemical deterioration of the retort metal and physical deforma- The' desired. temperature of from 2,200 to v2,500 de Vgrecs F. The covers I9 are removed to permit the placing of the charge I3, replaced, and the interior of the retort evacuated. Obyiously', the molten bath has a high specic heat so that the retort is not appreciably lowered in temperature |by the introduction of the charge. Moreover, the fluid has uniform contact with the entire portion of the retort inside the furnace; and as the fluid temperature is substantially uniform throughout or, under most extreme conditions, has a uniform point of heat transfer to the gas-metal obtaining tion due to unequal heating or intermittent operation must be considered as welll. All these factors must be reconciled if a proper retort is to be obtained. Experience has shown that present constructions employing a tubular or 'other shaped retort heated externally by flame or radiation or internally by resistance means or any of the other variations proposed will not suice. Referring to Figure 1, the retort constructed according to this invention is shown schematically. Essentially this consists of a furnace I0 having at least its lower portion constructed of uid-tight refractory material. Extending through. opposite walls II of the furnace is a metallic tubular retort I 2 preferably made of a substantially pure iron"y such as ingot iron, to avoid eutectic combinations which would lessen hightemperature strengths in which is placed the charge indicated as I3. The retort I2 is submerged in a molten bath |14 of a material which remains molten in the reaction range as, for example, sodium silicate, bariumchloride or calcium chloride which are fluid between 1,900 and 2,800 degrees F. The bath I4 is shown as heated directly by a gas flame I5 which is directed on it by a jet I6 which may be of the type used in glass furnaces. However, other means of heating the bath may be employed such as submerged glow bars, regenerative or recuperative furnaces,

elecrical conduction through the bath or othercommonly used industrial heat sources. A stack I1 is provided to remove the products of com-v bustion from the chamber of the furnace.

lIt will be noted that the retort I2 has ends I0 extending a considerable distance outside of the walls II and these are tightly sealed by covers I9. A vacuum connection 20 is provided on at gradient, so will be the temperature to which the retort is exposed. Finally, the contact between a fluid and metal is far superior from the standin prior furnaces.

It will therefore be found that due tothe uniformity of heating condition and speed of heat transfer, the time of reaction is considerably reduced.A Further, the same uniformity of temperature coupled with the uniformity of pressure exerted on the retort by the fluid in which it is immersed increases the effective retort life markedly. Still another advantage is that the protective character of the bath permits the use of ingot iron as a retort material which is less ex-` pensive and has a higher melting point and better heat-strength characteristics than any of the alloys previously used, but which could not withstand the usual furnace atmosphere. This is not limited to high-purity iron, however, for the usual retort metals may be used if desired with greatly improved life attributable mainly to the fluid immersion. v

While sodium silicate has been found very suitable as a bath, salts and other stable materials in the desired range may be used. Whatever may be the thermal insulating qualities of these materials in the solid state, they are found to be very good heat conductors whenr uid. In using the silicate bath, the high temperature appears to reduce the sodium content leaving a silica having a much higher'melting point. This can be controlled and the iiuidity maintained by addition of a sodium compound such as sodium carbonate or a like related material. The-depth of the bath should be adequate, of course, to cover the retort and the volume should be suii'icient to provide the heat content necessary for the maintenance of substantially uniform temperatures throughout the bath. The bath should be substantially pureor at least should not include substances such as sulphur or phosphorus which might react with the retort metal forming an eutectic having a lower melting point with consequent deterioration of the retort under heat. By using substantially pure iron having, as example, the following compositionz' Mn .015 FE Substantial balance and heating the retort made therefrom in the medium of a molten bath, the reasonable life expectancy of the retort will be measured in months. Experiments with the best heat-resistant alloys available in the conventional furnace show. at best, a useful life of to 20 days. And this saving is actually accompanied by a reduction in reaction time.

A further advantage is that the weight o! tube and charge may be proportioned to the volumetric displacement in the bath so that the tube and charge, in effect, iloat in it. Thus, at the high reaction temperature obtaining, the tube mayv iloat in the bath and be free from structural sup porting stresses. On the other hand, at unloading and loading the temperatures are somewhat lower and the strength of the retort material corf respondingly higher so that it will stand the fiotational forces.

Another advantage is that provision may be Ybetter witnmade to protect the interior of the tube as weil.

Thus, while the interior is under vacuum during the reaction and hence will not oxidize, during loading and unloading-it is open to the atmosphere and, of course, still at very high temperatures. To deal with this condition, the interior heat said bath to the reaction temperature, said tube being formed oi' substantially pure iron.

2. ,In a reaction furnace. an enclosurehaving oppositely disposed walls andA constructed ofrefractory materials, a tubular retort i'ormed of substantially pure iron normally readily oxidizable at the reaction temperatures supported on said walls extending through said enclosure and -having'a portion extending therefrom, said portion in said enclosure serving sa a reaction chamber, said extending portion serving as a condensing chamber i'or the product of said' reaction; a closure at the end of said retort, means to evacuate the interior of said retort, a bath in said enclosure in contact with the reaction portion of said retort and comprising molten sodium silicate substantially nonreactive with said retort material, and means to control oxidation of the in- 'terior of the retort by establishing a nitrogen atmosphere therein operable when said closure is removed. 1

3. The method oi' prolonging the eiiective life of lsubstlalntially pure iron retorts subjected to reoi.' the tube may be coated with a thin nonoxidiz able coating of nichrome or the like, or it may be given one of anumber of other available surface treatments`which will retard oxidation. As an alternative, an inert atmosphere such as nitrogen may be maintained in the tube when it is opened to reduce oxidation.

It is believed that thetoregoing clearly show the advantages inherent in this method and apparatus.' Longer lite from retorts made of less costly materials, greatly reduced operating time. increased safety, larger yields-these and others are the advantages which accrue. l

Some changes may be made in the arrangement, construction and combination of the several parts comprising the improved device without departing from the spirit of the invention, and it is the intention to cover by the claims such changes as may reasonably be included within 4 the scope thereof.

The invention claimed is:

1. In a reaction furnace. an enclosure, a metal retort tube supported within said enclosure and sistance .to surface oxidation in oxidizing atmos having a portion extending therefrom, a bath in said enclosure submerging said tube, said bath comprising molten sodium silicate. and means to pheres at such temperatures which comprises the steps o! -submerging the exteriors of said tubes into molten sodium silicate, heating said sodium silicate to the desired temperature, and periodif cally treating said bath' to prevent the formation of higher melting point silicas.

5. VI'he method of-protecting the inner and outer surfaces of substantially pure iron tubular retorts under high temperature, which comprises l the steps of maintaining molten sodium silicate on the exterior of the tube and/a nonoxidizing gaseous atmosphere in the interior-of said tube.

maaar n. man. ram o. saws. 

